The present invention relates to an optical data recording system and a method of production of a recording medium, or more in particular to an optical system for an optical data recording medium using linearly polarized light as a data reading light such as a magnetooptical recording medium drive system and a transparent substrate suitable especially for an optical data recording medium for reading data by use of linearly polarized light, and a method of production thereof.
In recent years, an optical data recording medium has been closely watched as a large-capacity recording medium comprising a magneto-optical recording film such as a phase-change type recording film or write-once type optical recording film or a reflection film on one side of a transparent substrate which is exposed to a light beam for recording and/or reproducing data. A linearly polarized light or circularly polarized light is used as a recording/reproducing light beam for an optical data recording medium of this type. (See "Analysis of Optical Anisotropy of Polycarbonate Substrate for PhotoMagnetic Recording", by Akihiko Yoshizawa, October 1986 issue of Kogaku).
The linearly polarized light or circularly polarized light, once transmitted through a birefringent medium, becomes an elliptically polarized light, and one of the two components of the linearly polarized light with directions of vibration perpendicular to each other is retarded from the other component. As a result, in the case where the transparent substrate is made of a birefringent medium, the normal process of reading data is adversely affected by the retardation, thereby deteriorating the S/N ratio of the detection signal. Especially in a magneto-optical recording medium, a magneto-optical recording film is formed on one side of a transparent substrate, and data is read out by detecting a change in Kerr rotation angle (direction of rotation of the polarization plane) of the parts formed and not formed with a reversed domain on the magneto-optical recording film due to the linearly polarized light.
In the case where the transparent substrate is a perfectly optical isotropic body without any birefringence, no special problem of data reading is posed regardless of the direction in which the polarization plane of the linearly polarized light making up a data reading light is placed. If the transparent substrate is an optical anisotropic body or is accompanied by birefringence, however, the linearly polarized light entering the transparent substrate becomes an elliptically polarized light while being transmitted therethrough, and the retardation directly acts as a noise component of the detection signal, thereby deteriorating the S/N ratio of the detection signal greatly, except in the case where the polarization plane of the linearly polarized light is set in a specific direction.
In order to reduce or eliminate the deterioration of the S/N ratio of the detection signal by retardation, research efforts have so far been made to prepare a transparent substrate with as small a retardation as possible by selecting a substrate material or conceiving a method of preparing the substrate. As a consequence, a conclusion was reached that a thermoplastic resin such as a polycarbonate resin, though high in productivity, is not suitable as a transparent substrate material for a magneto-optical recording medium, and a thermosetting resin such as an epoxy resin or ceramics such as glass has been suggested in place as a proper material of the transparent substrate.
Ceramics such as glass, however, have disadvantages that (1) cracking or cut is liable to occur so easily that it is difficult to handle during production, use or transport, and (2) a signal pattern such as prepit corresponding to an address signal or a guide track corresponding to a tracking signal is required to be processed through a method of applying a resin replica film transferred from a die by what is called the photo polymerization process, resulting in a low productivity. The problem of a thermosetting resin such as epoxy resin, on the other hand, is an extremely low productivity as compared with the thermoplastic resin to which the injection molding can be applied, in view of the fact that the forming thereof is impossible by other than the casting method (in a which resin in fluid state is injected into a die of predetermined shape and set by heating).
In addition, no matter how small a retardation of a material may be, it is as a matter of fact impossible to obtain a transparent material zero in retardation. Therefore, the conventional technical idea of selecting and using a transparent substrate material low in retardation has an inherent shortcoming that the S/N ratio is unavoidably reduced with the magnitude of retardation of the material.